1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack, a heat exchanger, a reformer, and a casing containing the fuel cell stack, the heat exchanger, and the reformer.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or the air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
As this type of fuel cell, for example, an all-in-one, space saving compact fuel cell power generation apparatus with small heat radiation loss is disclosed in Japanese Laid-Open Patent Publication No. 10-92457 (hereinafter referred to as the “conventional technique 1”). As shown in FIG. 18, the fuel cell power generation apparatus according to the conventional technique 1 includes a fuel preheating device 1, a reformer 2, a fuel cell 3, catalyst combustors 4, and a pressure container (not shown) containing the fuel preheating device 1, the reformer 2, the fuel cell 3, and the catalyst combustors 4.
The reformer 2 is stacked on the fuel cell 3. The fuel preheating device 1 and the catalyst combustors 4 are provided around the fuel cell 3. The fuel cell 3 and the reformer 2 are sandwiched between upper and lower tightening plates 5a, 5b. Components between the tightening plates 5a, 5b are compressed, and tightened together using a plurality of connecting rods 6 to apply a predetermined surface pressure to the fuel cell 3 and the reformer 2.
In the conventional technique 1, the fuel preheating device 1 and the reformer 2 are connected by a fuel gas pipe 7a and a reformed gas pipe 7b. The fuel preheating device 1 and the fuel cell 3 are connected by a reformed gas pipe 7c. Further, the fuel cell 3 and the catalyst combustors 4 are connected by anode exhaust gas pipes 7d and cathode exhaust gas pipes 7e. The catalyst combustors 4 and the reformer 2 are connected by combustion gas pipes 7f. 
As described above, in the conventional technique 1, a plurality of pipes including the fuel gas pipe 7a are provided. Thus, the thermal efficiency is lowered due to heat radiation from the pipes. Further, though the fuel cell 3 and the reformer 2 are provided in parallel, the fuel preheating device 1 and the catalyst combustors 4 are provided on sides of the fuel cell 3. Thus, the overall size of the fuel cell power generation apparatus is large.
In Japanese Laid-Open Patent Publication No. 2003-229164 (hereinafter referred to as the “conventional technique 2”), a solid oxide fuel cell system is disclosed. The solid oxide fuel cell system is proposed in an attempt to reduce the size, and reduce the heat loss as much as possible. As shown in FIG. 19, the solid oxide fuel cell system of the conventional technique 2 includes a solid oxide fuel cell (SOFC) stack 1a, a catalyst combustion all-in-one type heat exchanger 2a, and a pre-reformer 3a arranged vertically in a heat insulating container 4a. 
The catalyst combustion all-in-one type heat exchanger 2a includes a first heat exchanger 2b provided on the pre-reformer 3a, a catalyst combustion layer 2c provided on the first heat exchanger 2b, a second heat exchanger 2d provided on the catalyst combustion layer 2c. 
The fuel flows along a fuel supply line 5c, and the reformed fuel gas is supplied to the SOFC stack 1a. Air as the oxygen-containing gas flows along an air supply line 6a, and is supplied to the SOFC stack 1a. The exhaust fuel is discharged from the SOFC stack 1a through an exhaust fuel line 7g. The exhaust air discharged from the SOFC stack 1a is supplied to an intermediate point in the exhaust fuel line 7g through an exhaust air line 8.
In this structure, the exhaust fuel and the exhaust air from the SOFC stack 1a are supplied to the catalyst combustion layer 2c through the exhaust fuel line 7g and the exhaust air line 8. Then, the combustion gas combusted at the catalyst combustion layer 2c flows from the first heat exchanger 2b toward the pre-reformer 3a, and is used as a heat source for heating the pre-reformer 3a. 
The fuel flows toward the pre-reformer 3a through the fuel supply line 5c to generate a reformed gas. Heat exchange between the reformed gas and the combustion gas is performed at the first heat exchanger 2b. Then, heat exchange between the reformed gas and the exhaust air is performed at the second heat exchanger 2d. Thereafter, the reformed gas is supplied to the SOFC stack 1a. 
However, in the conventional technique 2, the combustion gas (the combusted exhaust fuel and the combusted exhaust air) is utilized as a heat source for heating the pre-reformer 3a. Thus, the pre-reformer 3a can be damaged easily. The combustion gas has a significantly high temperature, and contains water vapor. Therefore, the combustion gas oxides the pre-reformer 3a easily, and lowers the durability. For this reason, the pre-reformer 3a is made of highly antioxidative material, which is expensive and uneconomical.